Creping blades are commonly used in the paper industry for production of tissue. In order to produce the typical bulk characterizing creped tissue, a creping blade is normally used for detaching a paper web from a rigid, hot dryer cylinder (often known as a Yankee dryer) and at the same time exert a compressive action on the paper web.
In this context, there are a number of properties which are desired for the creping blades. The creping blade should be able to overcome the adhesive forces which stick the paper web to the dryer surface. At the same time, the blade should create the desired crepe structure in order to provide the right bulk, softness and mechanical strength to the tissue. For this purpose, the geometry of the blade tip plays an important role. For example, a square edge blade (i.e. 90 degrees bevel) in any given creping situation will create a different tissue than a blade having a sharp edge of, say, 75 degrees bevel under otherwise similar conditions. The square edge blade would, in this example, provide a higher bulk and a coarser crepe structure than the 75 degrees blade.
In addition, and not less importantly, the blade should be able keep the tissue parameters as constant as possible for the longest possible period of time, in order to produce tissue of substantially constant quality. Wear and other damages to the blade tip are therefore important factors determining the quality of the final tissue product, as well as the service life of the blade.
Creping blades are subjected to wear for a number of reasons. For example, there will be sliding wear against the dryer, and there will be impact wear on the blade due to the paper web hitting the blade during creping. It has been found that the progressive wear of the creping blade is directly related to unwanted evolution of the tissue properties, such as changes in bulk or softness. In practice, optimal properties are obtained only with a newly installed blade.
In order to accommodate for the wear of the creping blade, tissue manufacturers are typically specifying ranges of properties which are deemed to be acceptable. Nevertheless, it would be highly appreciated in the tissue industry if the quality obtained during the initial time after a blade change could be maintained for a prolonged period of time.
One type of damage occurring in creping blades is chipping at the working edge of the blade. By chipping, it is meant that small chips of blade material at the blade edge are torn off during creping. Chipping is typically a limiting factor for blades having a hard-covered edge, such as an edge covered with a ceramic, a carbide, a cermet or some other hard, wear-resistant material. If they are relatively small, such chips at the blade edge are responsible for defects sometimes referred to as lines or “tramlines”. For larger chips, or for lower grammage of tissue, such chips may cause web breaks and holes in the tissue, with a considerable loss in productivity as the result.
In order to reduce such chipping at the blade edge, it has previously been proposed to provide the blade with a thermally sprayed top layer that forms a working edge, a sliding wear area and a web impact area, wherein the top layer comprises both chromia and titania (see WO2005/023533).
However, a more general solution to the above-referenced chipping issue is still sought. In particular, it would be highly advantageous if a solution to the chipping issue could be provided that is largely independent of particular material selections.